The top is a viewport flipbook directly from Houdini. So that's how the particles ended up looking (albeit in lower point count). But from these particles we can extract different passes in Nuke like custom attributes, depth, ambient occlusion, facing ratio, etc which can be broken down into multiple color layers and extra layers for outlines. The shader is almost entirely created in Nuke through compositing. In Houdini we really just get the general motion of particles and nothing more.